Warning: napkin math incoming. Anyone with metalworking/auto building who can refute my numbers, please speak up!
According to the "How It's Made: Dream Cars" episode on the Model S, the "body in white" of the Model S is 410lbs of aluminum. I have no idea how much is "wasted", but figure ~30%. So, let's call it 550lbs. At Wednesday's commodity price for aluminum of
$0.68/lb, 550lbs of aluminum costs $375. 550lbs of steel is $149 at current price of
$540/US Ton (this is a number I found for cold-rolled steel; it's much harder to find a price for this than aluminum) - but that's not really helpful, since steel is ~2.5x denser than aluminum. So, that 550lbs becomes 1375lbs, for a cost of $371!
So, basically no difference in cost - and less retooling/retraining for Tesla's current manufacturing capability. And who knows how much more battery would be needed to move the additional weight with sufficient range? It may not be completely unreasonable for larger amounts of aluminum to be used than originally expected.
Now, somebody needs to check on my assumptions re: steel density/prices/how much steel you'd need to cover the same amount of body area. I have a bad feeling I'm missing something....but I've looked it over a couple of times and can't find it.